U.S. Pat. No. 3,057,720 suggests that two color toner images can be formed consecutively on the same image frame without fixing the first image if the second toning step is not so harsh as to clean off the first toner image. Three color systems are disclosed using either positive development or discharged area development. A variety of development methods are suggested.
Japanese Kokai 56-144452 (1981) also discloses a process in which two or three color toner images are formed on the same frame of a photoconductive member. As disclosed, the photoconductive member is uniformly charged, exposed and reverse developed with a toner of a first color. With the unfixed toner electrostatically adhering to the exposed areas the photoconductive member is exposed to a second image that does not overlap with the first image. That image is then developed by application of a second color toner to the newly exposed areas. The process can be repeated for more colors. The resulting multicolor image is transferred to a receiving sheet in one step.
These processes can be set up to double the speed of prior two color processes in which separate images are made on separate frames and then transferred in registration. Further, the required registration of the exposing steps is far easier to accomplish with accuracy than is registration in the transfer step.
However, this process has an inherent problem when used with conventional magnetic brush systems that the second and subsequent toning steps have a tendency to scrape or brush off the toner applied in the earlier toning steps. The above disclosure is representative of many which suggest the only way to solve this problem is to leave a gap between the brush and the image. By any of a number of approaches suggested in the prior art the toner (from both mono-component and dual-component developer) is propelled or projected across the gap to develop the electrostatic image without brushing off the prior formed toner images. See U.S. Pat. No. 4,629,669; European Patent Application No. 0240888; European Patent Application No. 0066141; U.S. Pat. No. 4,599,285 and U.S. Pat. No. 3,775,106.
However, projection toning brings its own set of problems to the system. The most serious is a difficulty in doing high density toning at reasonably fast machine speeds. Also, the gap between toner and image is critical and must be maintained. Toners usable in the process are limited, limiting the colors available; especially difficult to use in this process are those of lighter hue, such as yellow. It is a difficult process to establish a background controlling electrical field.
U.S. Pat. Nos. 4,546,060; 4,473,029 and 4,531,832 describe a method of toning in which a magnetic brush applicator supplies two component developer including small hard magnetic carrier particles and electrically insulative toner to an image which moves past the development station at a predetermined velocity. The brush includes a rapidly rotating core which tumbles the hard carrier particles through a development zone. The tumbling of the carrier is apparently due to the changing magnetic field which continuously flips the carrier. This process has a number of advantages in image quality, and is being used commercially.